Fuel cells are devices that directly convert chemical energy of reactants, i.e., fuel and oxidant, into direct current (DC) electricity. For an increasing number of applications, fuel cells are more efficient than conventional power generation, such as combustion of fossil fuel and more efficient than portable power storage, such as lithium-ion batteries.
In general, fuel cell technologies include a variety of different fuel cells, including alkali fuel cells, polymer electrolyte fuel cells, phosphoric acid fuel cells, molten carbonate fuel cells and solid oxide fuel cells. Today's more important fuel cells can be divided into three general categories, namely fuel cells utilizing compressed hydrogen (H2) as fuel, proton exchange membrane (PEM) fuel cells that use methanol (CH3OH) reformed into hydrogen as fuel, and PEM fuel cells that use methanol (CH3OH) fuel directly (“direct methanol fuel cells” or DMFC). Compressed hydrogen is generally kept under high pressure, and is therefore difficult to handle. Furthermore, large storage tanks are typically required, and cannot be made sufficiently small for consumer electronic devices. On the other hand, fuel cells using methanol reformats require reformers and other vaporization and auxiliary systems thereby increasing the size and complexity of methanol-reformat based fuel cells. DMFC is the simplest and potentially smallest fuel cell, and holds the most promising power application for consumer electronic devices.
DMFC for relatively larger applications typically comprises a fan or compressor to supply an oxidant, typically air or oxygen, to the cathode electrode, a pump to supply a water/methanol mixture to the anode electrode and a membrane electrode assembly (MEA). The MEA typically includes a cathode, a PEM and an anode. During operation, the water/methanol fuel liquid mixture is supplied directly to the anode, and the oxidant is supplied to the cathode. The chemical-electrical reaction at each electrode and the overall reaction for the fuel cell are described as follows:
Reaction at the anode:CH3OH+H2O→CO2+6H++6e−
Reaction at the cathode:O2+4H++4e−→2H2O
The overall fuel cell reaction:CH3OH+1.50O2→CO2+2H2O
Due to the migration of the hydrogen ions (H+) through the PEM from the anode through the cathode and due to the inability of the free electrons(e−) to pass through the PEM, the electrons must flow through an external circuit, which produces an electrical current through the external circuit. The external circuit may be any useful consumer electronic devices, such as mobile or cell phones, calculators, personal digital assistants and laptop computers, among others. DMFC is discussed in U.S. Pat. Nos. 5,992,008 and 5,945,231, which are incorporated by reference in their entireties. Generally, the PEM is made from a polymer, such as Nafion® available from DuPont, which is a perfluorinated material having a thickness in the range of about 0.05 mm to about 0.50 mm. The anode is typically made from a Teflonized carbon paper support with a thin layer of catalyst, such as platinum-ruthenium, deposited thereon. The cathode is typically a gas diffusion electrode in which platinum particles are bonded to one side of the membrane.
One of the most important features for DMFC application is fuel storage. Another important feature is to regulate the transport of fuel out of the fuel cartridge to the MEA. To be commercially useful, DMFC systems must have the capability of storing sufficient fuel to satisfy the consumers' normal usage. For example, for mobile or cell phones, for notebook computers, and for personal digital assistants (PDAs), fuel cells need to power these devices for at least as long as the current batteries, and preferably much longer. Additionally, the DMFC should have easily replaceable or refillable fuel tanks to minimize or obviate the need for lengthy recharges required by today's rechargeable batteries.
The patent literature contains no specific discussion of non-pressurized portable fuel tank or fuel storage for fuel cells. U.S. patent application publication no. U.S. 2002/0127451 A1 discloses a compact PEM fuel cell that stores methanol fuel in upstanding circular tank(s) and vents the by-product CO2 back into the tank to pressurize same. This fuel tank further comprises a release valve to prevent the over-pressurization of the tank and a fuel intake valve to add fuel. The fuel tank comprises a porous layer to wick the water/methanol fuel mixture by capillary action to the anode terminal of the PEM. However, this porous layer cannot remain in contact with the fuel in positions other than vertical or at a slight angle from vertical. Hence this fuel tank cannot be used in all orientations.
Similarly, U.S. patent application publication no. 2001/0051293 A1 discloses a wicking structure made from an absorbent material in fluid communication with a refillable fuel reservoir. The wicking structure's function is to bring fuel to the PEM by capillary action in regulated amounts. However, this reference does not describe the method for regulating fuel flow, or how the wicking structure maintains contact with the fuel when the fuel level is less than full for capillary action to work.
U.S. Pat. No. 6,326,097 B1 discloses, among other things, fuel ampoules that can be filled with fuel permeable materials that allow the fuel to be communicated via capillary action in any orientation to a fuel needle to be wicked to the PEM. These fuel ampoules cannot store a sufficient amount of fuel, because for capillary action to work properly the spacing within the permeable materials is necessarily small. Hence, the fuel permeable materials take up most of the space in the ampoules, thereby reducing the storage capability. This reference also discloses a hand-operated pump, i.e., a dimpled area on the ampoules, for the user to push to pump fuel. This pump is also impractical since it requires the user to pump before power can be supplied to the electronic devices, and may require the user to continually pump the fuel cell to maintain the flow of fuel to the PEM. Additionally, each hand pumping action may send a surge of fuel to the PEM, and may cause an undesirable surge in the electrical output from the fuel cell to the electronic devices. Importantly, the '097 reference provides no teaching as to how the unused fuel absorbed by the permeable materials can be transported to the PEM.
U.S. patent application publication no. 2002/0018925 A1 discloses a cavity in an electronic device, where a balloon containing fuel is stored or where an absorbing solid containing fuel is stored for use with a fuel cell. Similar to the '097 reference, this absorbing material would take up most of the space in the fuel tank and would retain fuel within the absorbing materials, thereby reducing effective fuel storage capacity.
U.S. Pat. No. 6,447,941 B1 discloses a plurality of horizontal fuel permeating layers that are in contact with fuel in a fuel tank, and the fuel is communicated by capillary action from the fuel storage to the fuel permeating layers. The fuel is then evaporated in fuel evaporating layers before reaching the anode terminal. This fuel tank does not have any internal structure to aid in the transport of fuel.
U.S. Pat. No. 6,460,733 B2 discloses a multi-walled fuel container comprising an inner container of a methanol fuel disposed inside an outer container. The inner container may have rigid walls or may be a distensible bladder. The plenum area between the two containers comprises agents or additives that neutralize the methanol fuel in case of breakage or before disposal. The fuel is fed to a fuel reservoir or directly to the anode electrode by gravity or by a pressurized gas source located within the outer reservoir. An external pump is provided to communicate the fuel to the PEM.
U.S. Pat. Nos. 5,709,961 and 6,268,077 B1 disclose pressurized fuel tanks to communicate fuel to the fuel cell.
Hence, there remains a need for a fuel storage device that possesses high storage capacity and does not require a pressurized source to transport the fuel to the PEM from the storage device.